Page 118 - Multifunctional Photocatalytic Materials for Energy

Carbon nitride photocatalysts 107

the sample prepared at 520°C was higher than those synthesized at 470°C, 500°C,
and 540°C because of the synergistic effect of the BET area, surface defects, and the
visible light absorption threshold.
As a low-cost, environmentally benign, and N-rich precursor, urea is also uti-
lized in the preparation of graphitic carbon nitride. In a previous work, Zhang and
coworkers [23] reported that calcination of urea at 550°C for 3 h without any assis-
tance can produce a highly efficient photocatalyst of g-C 3 N 4 . BET results indicated
2 −1
that a larger surface area (69.6 m g ) was obtained on urea-derived carbon nitride
2 −1
2 −1
than on thiourea-derived (11.3 m g ) and dicyanamide-derived (12.3 m g ) carbon
nitride. The same results were observed in a hydrogen evolution reaction with an en-
−1
hanced H 2 production rate of 47.2 μmol h , which was 3.1 times higher than that of
thiourea-derived carbon nitride and 2.26 times to that of dicyanamide-derived carbon
nitride. The polymerization temperature, the heating rate in condensation of carbon
nitride, and the co-catalyst are also of vital importance in terms of the amount of H 2
generation. In another report, Tang and coworkers [24] investigated H 2 evolution rates
using urea-based carbon nitride synthesized at different polymerization temperatures,
heating rates, categories, and quantities of co-catalyst. An extraordinary performance
−1 −1
(c.20,000 μmol h g ) and a high turnover number (more than 641 after 6 h) were
observed on the optimized photocatalyst. Further experimental and DFT studies found
that the high H 2 production rate was ascribed to the higher polymerization and the
lower proton concentration.
In addition to melamine and urea, thiourea is another good candidate for synthesis
of carbon nitride. Zhang et al. [18] successfully prepared graphitic carbon nitride by
heating thiourea at 550°C in the air. Fig. 6.2C shows the self-polymerization of the
thiourea scheme. Similar to the polymerization process for cyanamide, dicyanamide,
and urea, melamine is also the intermediate when using thiourea as the precursor. In
addition, the influence of heat treatment was investigated by keeping the temperature
between 450°C and 650°C. BET results showed that more active sites were obtained
because of the formed nanostructure, and the UV-vis spectra showed a blue shift be-
cause of quantum confinement effects. For water splitting, the hydrogen generation
−1
rate of thiourea-derived carbon nitride at 650°C can reach as high as 157.2 μmol h .
Acid pretreatment of precursors as a valid means of altering the physiochemical
properties of carbon nitride photocatalysts has been of particular interest. Yan and
coworkers [25] reported the preparation of graphitic carbon nitride via direct thermal
polymerization of H 2 SO 4 -treated melamine. Because the sublimation of melamine
during polycondensation was efficiently suppressed by sulfuric acid, a lower polymer-
ization degree of the amino groups in pretreated melamine and a higher BET surface
area were achieved, compared with pristine melamine. As a result, the rate of hydro-
gen evolution on carbon nitride via sulfuric acid pretreatment is twice that of untreated
carbon nitride.
Table 6.1 provides a summary of carbon nitride photocatalysts prepared using dif-
ferent precursors and different polymerization parameters and their corresponding
BET areas and H 2 production rates in water splitting. It can be concluded that most
urea-derived pristine carbon nitride photocatalysts possess larger surface areas than
those polymerized via other precursors, which makes the former attractive in terms of